The molecular mechanisms governing the regulation of the drug-detoxifying enzyme, UDP glucuronosyltransferase (transferase), and the structural differences between members of this family are being investigated in the rat. This animal, as exemplified by the Gunn rat, provides the only known animal model for investigating the defect in the glucuronidation of bilirubin and certain xenobiotics, characteristic of the Crigler-Najjar syndrome in humans. Three cDNA clones encoding different forms have been isolated and sequenced. The mRNAs complementary to two of these clones (pUDPGTr-1 and pUDPGTr-3) are 85% similar by sequence analysis and are not elevated by 3-methylcholanthrene or phenobarbital treatment. They appear to be transcribed from genes belonging to the same subfamily. The mRNA complementary to the third clone, pUDPGTr-2, is only 65% similar in sequence to the other cDNAs. It is elevated 5-fold by phenobarbital and appears to be transcribed from a gene belonging to a different subfamily. Sequence studies have also shown that the transferase forms encoded by the three cDNAs contain signal peptide and membrane anchoring regions, and potential asparagine-linked glycosylation sites. In vitro translation studies indicate that the signal sequence is most likely cleaved during insertion into the endoplasmic reticulum. Experiments are in progress to express those transferase cDNAs which contain complete coding regions in transferase-deficient cells, in order to characterize each form by its catalytic activity and substrate specificity. The regulation of each form--as a function of age, tissue distribution and administration of prototypic inducers--will also be investigated.